Borderline personality disorder (BPD) is extremely debilitating and costly to the individual and larger community. Additionally, BPD is highly comorbid with alcohol use disorder with prevalence estimates around 50% and an even higher percentage of individuals with BPD experiencing alcohol-related problems. This comorbidity significantly worsens treatment outcomes. To date very little research has examined mechanisms that may underlie both BPD and alcohol-related problems. The proposed project examines two hallmark features of BPD, affective instability and impulsivity/disinhibition, which have also been widely studied in relation to alcohol problems to test two pathways for why individuals with BPD engage in heavier alcohol use and experience more problems while drinking. Importantly, the disinhibition and anxiolytic/stress reducing effects of alcohol have been modeled successfully in the laboratory and our current proposal utilizes cutting edge research methodology to examine this important topic. Aims: The current application tests hypotheses about differences in response to alcohol both between groups comparing individuals with and without BPD and intra-individual differences in these responses as predictors of heavy alcohol use and alcohol-related problems. Specifically, we are hypothesizing that individuals with BPD will have heightened sensitivity to the anxiolytic effects of alcohol which in turn will increase their engagement in heavy drinking behaviors as a way to regulate their affective instability. We are also hypothesizing that individuals with BPD will be more sensitive to the disinhibiting effects of alcohol compared to individuals without BPD and that this acute response will partially account for the higher number of alcohol-related problems seen in this population. Further, by examining individual differences in response to alcohol we are able to examine a continuum of these constructs which takes into account the notable heterogeneity of individuals with and without BPD. Approach: We will use a tightly controlled within-subjects laboratory alcohol administration to model change in stress response and disinhibition from sober to intoxicated. Young adult drinkers (21-30 years of age; 50% female) with and without a diagnosis of BPD will be recruited to participate (N = 100; 50 BPD and 50 non-BPD) from both the community and treatment clinics. All participants will complete Axis I and Axis II semi-structured interviews, a control and alcohol lab session, and a three-month timeline followback interview to allow us to examine how lab-based response to alcohol relates prospectively to heavy alcohol use and alcohol-related problems. In accordance with NIAAA strategic priorities on identifying mechanisms of comorbidity the proposed work takes a critical step by increasing our understanding of why BPD and alcohol-related problems are highly linked and will ultimately inform future intervention efforts designed to decrease problematic drinking behaviors for this population.